tOO II. S. WASHINGTON — IGNEOUS COMPLEX OF MAGNET COVE 



approaches the leucite-porphyry, though a little lower in Si0. 2 and alka- 

 lies, and higher in MgO and CaO. It differs mineralogically in that 

 orthoclase replaces leucite, and hornblende and titanite replace garnet. 

 It is thus another example to be added to the rapidly growing list of 

 magmas which are closely similar in chemical composition, but which 

 form on solidification diverse mineral aggregates. 



In the hand specimen the rock shows considerable resemblance to the 

 shonkinites of Pirsson.* They also resemble these under the microscope, 

 though in the shonkinites augite is more abundant, biotite replaces 

 hornblende, and there is practically no nepheline. Corresponding to 

 these differences the shonkinites are more basic, MgO and CaO being 

 much higher and A1. 2 3 and alkalies lower. At the same time the two 

 evidently belong to the same type of magma, so that for the sake of con- 

 venience this fine grained syenite may be called shonkinite. It is closely 

 similar chemically to the essexites and theralites, though it differs min- 

 eralogically, as it contains neither plagioclase nor olivine. It may be 

 noted that Rosenbusch f speaks of essexite as occurring in Arkansas, 

 without specifying the locality. He possibly refers to this rock. 



IV is of a fairly typical specimen of Williams' " Ridge-type of eleolite- 

 garnet-syenite." It is dark and rather coarse to rather fine grained, 

 with a granitic structure. It is composed of fresh nepheline in large 

 amount, with much pale green or }^ellow diopside, and xenomorphic 

 brown garnet, which is younger than the pyroxene. Apatite is present 

 in rather large crystals, while magnetite varies, being rare in some speci- 

 mens and abundant in others. Hornblende, olivine, orthoclase, and 

 plagioclase are absent. 



This rock corresponds very closely both chemically and mineralogically 

 with the ijolite described by Ramsay and Berghellj; from Iiwaara, in 

 Finland, and is therefore to be recognized as another occurrence of this 

 very interesting type. 



Y is of Williams' " el eolite- mica-syenite (light Cove type)," a coarse 

 grained rock of granitic structure, the color being in general pinkish or 

 yellowish, mottled with black. It is composed of nepheline, brown 

 garnet (both melanite and schorlomite), some biotite and diopside, a 

 little orthoclase, and titanite, magnetite, and apatite. There is neither 

 hornblende, olivine, nor plagioclase present. This rock may be called 

 biotite-ijolite. 



Judging from the specimens which I collected, these two rocks, IV 

 and V, do not differ materially, though the latter does contain some 



* Weed and Pirsson : Hull. Geol. Soe. Amer., vol. vi, 1895, i>. 389. 



+ Rosenbusch : El em. Gesteins lehre., p. 173. 



J Ramsay and Berghell ; Geol. For. Fdrh. Stoek., vol. xii, 1891, p. 3U0. 



