454 



Scientific Geology. 



The most elegant variety of porphyritic sienite that I have met with 

 in the State, occurs in Abington, and North Bridgwater, and in other 

 parts of Plymouth county. Its base consists of quartz and feldspar, 

 with an abundance of epidote disseminated and in veins. The feld- 

 spar crystals that constitute it a porphyry, are of a flesh color. There 

 is also a dark colored mineral diffused through the mass, which may 

 be hornblende or mica. This rock if polished would form it seems to 

 me the most elegant ornamental stone in the State. (Nos. 1344 to 1347.) 



The sienite of Cape Ann is often porphyritic. In one place, 

 about half way between Sandy Bay and Gloucester Harbour, I found 

 a variety in which the imbedded feldspar crystals are of a very rich 

 bronze color, approaching in appearance to hypersthene. But when 

 this rock is smoothed its aspect is too dark to be elegant. (No. 1343.) 



5. Conglomerated Sienite. (Nos. 1350 to 1353.) This is a most in- 

 teresting variety on account of the bearing which its characters have 

 upon the theory of the formation of sienite. I have met with it chief- 

 ly in the compound No. 3, just described. The rock in general does 

 not differ from that variety : but it contains rounded masses of the old- 

 er stratified rocks. It is in fact a real conglomerate; and in some 

 places the nodules are so numerous that it has very much the aspect 

 of the course puddingstones of the newer rocks. The nodules vary 

 in size from the diameter of half an inch to that of six or eight inches. 

 They are not smoothed, like the pebbles in the more recent conglom- 

 erates, by mechanical attrition : but they appear like masses of rocks 

 that have been partly melted down by heat. In almost all cases horn- 

 blende predominates in these nodules : and often they consist of dis- 

 tinct hornblende slate. Sometimes they contain mica in considerable 

 quantity, and more rarely they consist chiefly of quartz and mica, the 

 former in excess, forming a kind of quartz rock. Feldspar is also 

 frequently present, especially in those cases where the schistose struc- 

 ture is indistinct ; and sometimes the nodule appears to be only a va- 

 riety of sienite in which the feldspar is in a smaller quantity than usu- 

 al. Upon the whole, I think 1 have ascertained the presence of horn- 

 blende slate, mica slate, and quartz rock, in these nodules. When 

 the rock is broken they are knocked out without difficulty, like the 

 pebbles from a common conglomerate. 



The theoretical inferences deducible from these facts I shall reserve 

 for the sequel. 



6. Augitic Sienite. The presence of hornblende in this rock and 

 the absence of mica, have led me to call it augitic sienite rather than 



