We don't like the clergy much, unless they're made of stone

He is rambling round this dark and probably echoing interior.  Vaulted stone is overhead. To describe what he sees he is using precise terminology, not all of which is penetrable, e.g. "The transept has no lateral naves."

This conveys nothing to me, and there is much more like it, so my eye quickly skims down to his next illustration.

Now, this I like.  This says gloom, and grandeur, and gothic. The pygmy human figures give scale to its magnificence while the strong Spanish sunlight plummets into the space beyond like a waterfall into a subterranean cavern.  Young ladies at home in Victorian parlours could sigh over this and wistfully imagine themselves as nuns, gliding about in a holy way with sexily downcast eyes.

Nathaniel would probably have taken a second look at a nun, but in general seems to find the clergy unimpressive, because here he drops in one of his little asides about a couple of quarrelling bishops:

"Don Pedro Fernandez de Frias, Cardinal of Spain... was, it is affirmed, of low parentage, of base and licentious habits of life, and of a covetous and niggardly disposition. These defects, however, by no means diminished the high favour he enjoyed at the successive courts of Henry the Third and Juan the Second. The Bishop of Segovia, Don Juan de Tordesillas, happened by an unlucky coincidence to visit Burgos during his residence there. The characters of the two prelates were not of a nature to harmonise in the smallest degree, and, being thrown necessarily much in each other's way, they gave loose occasionally to expressions more than bordering on the irreverent. It was on one of these occasions, that, the eloquence of the Cardinal Bishop here interred being at default, a lacquey of his followers came to his assistance, and being provided with a palo, or staff, inflicted on the rival dignitary certain arguments ad humeros—in fact, gave the Bishop of Segovia a severe drubbing. The Cardinal was on this occasion compelled to retire to Italy."

So much for undignified dignitaries.  But soon he comes across a clergyman he really does like - a carving decorating an arch.  It is a cowled head, perhaps meant to represent St. Francis :

"The attention is instantly rivetted by this head: it is not merely a masterpiece of execution. Added to the exquisite beauty and delicate moulding of the upper part of the face, the artist has succeeded in giving to the mouth an almost superhuman expression. This feature, in spite of a profusion of hair which almost covers it, lives and speaks. A smile, in which a barely perceptible but irresistible and, as it were, innate bitterness of satire and disdain modifies a wish of benevolence, unites with the piercing expression of the eyes in lighting up the stone with a degree of intellect which I had thought beyond the reach of sculpture until I saw this head. Tradition asserts it to be a portrait of Saint Francis, who was at Burgos at the period of the completion of the cathedral; and who, being in the habit of examining the progress of the works, afforded unconsciously a study to the sculptor."

He draws it, as you see: but by the time the head has passed through his hands and those of the artist who transferred his work into etchings, I can't see what's so great about it. Nor can I find the original in google images. Has it gone?