446 



Scie?itijie Geology. 



sents numerous varieties and shades' of color. One of the most -ele- 

 egant, is a light green, such as occurs in Chelsea and Maiden ; (Nos. 

 1254, 1256, 1257,) or the deeper green that I have met with in Mil- 

 ton. (No. 1255.) Red of various shades is a still more frequent color. 

 (Nos. 1247 to 1253,) A reddish brown is sometimes met with. (Nos. 

 1240, 1241, 1243, 1258.) A nearly black color more often : (Nos. 

 1234, 1235, 1236,) A gray color sometimes: (Nos. 1239, 1242, 

 1244,) and a purple color rather seldom. (Nos. 1232, 1233,) The 

 imbedded crystals are usually of a light color, sometimes white, some- 

 times brown, and sometimes greenish. Generally they are foliated, 

 very rarely compact, and distinguished from the base chiefly by the 

 color. 



3. Porphyry with a base of compact Feldspar and two or more min- 

 erals imbedded. (Nos. 1258 to 1262,) Feldspar and quartz are the two 

 minerals present ; but I have noticed several small plates of mica. It 

 is very obvious, indeed, that this rock is intermediate between sienite 

 and porphyry. In other words, it seems to be the former rock partly 

 melted down into the latter. The porphyry of the Blue Hills is chief- 

 ly made up of this variety ; though some perfectly formed porphyry 

 is found there. The quartz is usually hyaline and smoky, and some- 

 times it forms the only imbedded mineral ; the feldspar being all com- 

 pact. In such case especially, where the base is of a light color, the 

 rock exceedingly resembles trachytic porphyry; (Nos. 1261, 1262,) 

 and it will hardly admit of being polished for ornamental purposes. 

 That a porphyry, which, by being thus associated so intimately with 

 sienite, is proved to be one of the oldest varieties on the globe, should 

 so much resemble the most recent variety, proves that similar causes 

 have operated in its production at different and very remote periods. 



4. Brecciated Porphyry. I know not how to describe this variety 

 better, than by saying that it is composed of angular fragments of por- 

 phyry and compact feldspar, re-united by a paste of the same materi- 

 als, which is itself also porphyritic. Hence it appears that there must 

 have been an original formation of these rocks, (compact feldspar and 

 porphyry,) which was subsequently broken up, either by the mechan- 

 ical agency of water, or the mechanico-chemical agency of heat, re- 

 dissolving and mingling the materials. The fragments are of various 

 colors, usually however, gray or red : and this proves that rocks from 

 different localities must have been mixed together. The paste is com- 

 monly in the greatest quantity ; and the rock is as hard and broken 

 with as much difficulty as any other variety of porphyry. It is not 



