Varieties of Sie?iite. 



453 



and Cape Ann sienites, and the remarkable absence of mica, have led 

 me to associate it with the variety under consideration : and in fact it 

 forms a part of the same range. I distinguish this rock from granite 

 only by the absence of mica : yet it must be obvious that this mark is 

 not very satisfactory. 



Sometimes the feldspar in this compound of that mineral and 

 quartz, is nearly or quite compact. I have observed this variety most 

 frequently in the north part of Essex county, as in Rowley and New- 

 bury. A like compound is connected with the sienite of other coun- 

 tries. 



3. Feldspar, Hornblende, Quartz and Mica. In this quaternary 

 compound we have a still nearer approach to granite. And general- 

 ly it passes into granite by the disappearance of hornblende and the 

 increase of quartz and mica. Yet in all cases where I have noticed 

 it, this rock occupies a position between genuine granite and the newer 

 stratified rocks. Hence I infer, that geologically considered, the dif- 

 ference between it and granite are important to be noticed. 



The feldspar and hornblende are the predominant ingredients in 

 this variety. The quartz is in so small grains that it is apt to escape 

 notice ; and the mica, being usually black, is very easily mistaken for 

 hornblende. In general all the ingredients exhibit a liveliness of 

 crystalline structure which is observable only in the oldest rocks. . 

 The feldspar is ordinarily white, sometimes flesh colored, and the 

 hornblende black. The grain of the rock is commonly finer than 

 that of the 2d variety. As yet it has been but seldom employed for 

 architectural purposes ; although it would be beautiful and enduring. 

 (Nos. 1319 to 1340.) 



4. Porphyritic Sienite. (Nos. 1341 to 1349.) I mean by this term 

 any sienite through which are interspersed crystals or foliated masses 

 of feldspar, so as to give the rock a porphyritic aspect. I do not re- 

 collect, however, ever having seen the last variety above described, 

 porphyritic. And indeed, nearly all the rock which I regard as 

 porphyritic sienite in Massachusetts, and specimens of which will be 

 found in the collection, is almost entirely destitute of hornblende ; 

 and hence many geologists would regard it as porphyritic granite. 

 But the specimens are rarely wanting in veins and disseminated 

 masses of compact epidote, and I cannot but regard this mineral as 

 more decidedly characteristic of our sienite than hornblende. If this 

 be present and the mica almost or entirely absent, I have little hesita- 

 tion in regarding the rock as geologically a part of a sienitic deposit. 



