BERKSHIRE CAS^OK 841 



cal map as a propylite. It consists of a micro-crystalline groundmass of 

 feldspar and brown undecomposed hornblende, in which the feldspar 

 largely predominates. In this groundmass are imbedded clear, undecom- 

 posed, black and. brown hornblende, a few biotite flakes, and both ortlio- 

 clase and plagioclase, the latter slightly outnumbering the former. This 

 rock, however, has much the habit of a trachyte, and in reality represents 

 an intermediate condition, where, although plagioclase slightly predom- 

 inates and carries the rock under the name of propylite, at the same time 

 the proportion of orthoclase is so large as sensibly to affect its habit. 

 Geologically it appears to be a trachyte, but lithologically a propylite. 

 From its geological relations to the neighboring rocks, it was impossible to 

 arrive at any definite conclusion as to its place in the volcanic series. It 

 is younger than the melaphyrs, and older than the sanidin-trachyte which 

 overflows it to the north, so that its position in time, so far as known, would 

 coincide as well with propylite as with hornblende-plagioclase-trachyte. It 

 builds up conical hills 600 feet in height, which show no bedding planes 

 or other structural lines, and would appear to be a dense homogeneous 

 mass without indications of successive flow. 



Just to the south of these propylite hills is a series of dikes of light 

 greenish-gray rhyolite, which are seen to cut through the melaphyr and 

 to overflow the base of the trachyte hills. This rhyolite is an extremely 

 fine-grained rock, of a shaly structure, and is composed of a micro-crystalline 

 groundmass, in which are observed a few partially decomposed orthoclase 

 crystals and a little altered mica. 



A short distance farther down the canon, apparently breaking through 

 the propylite body, is a broad dike, several hundred feet in width, of typi- 

 cal hornblende-andesite. Brown, unaltered hornblende, prevailing plagio- 

 clase, many crystals of sanidin, and a few biotite flakes are seen everywhere 

 dispersed through the grayish-brown groundmass. Perhaps this rock should 

 be classed with the above-described propylites, for it only differs from 

 them mineralogically in the proportion of sanidin and an increase of horn- 

 blende. It- is noticeable that the fragments of hornblende lie for the most 

 part in parallel layers through the groundmass. Unlike the propylite, how- 

 ever, it has the true geological habit of andesite, w ith the peculiar smooth, 



