42 C, P, SHEIBNER ON FOYAITH, AN ELAOLITIC 
2, On Foyarrz, an Kimourtic SyENrTE occurring in PortuGaL. By 
Cuartes P. Suersnur, Esq., Ph.D., F.G.S., Assoc. Inst. C.E. 
(Read May 22, 1878.) 
[Puatss I. & II.] 
Tue name of this rock is derived from the locality where it occurs, 
viz. from Mount Foya, in the south of Portugal. It is cursorily 
referred to as granite in Charles Bonnet’s description of the Province 
of Algarve ; and as early as 1861 specimens of it were submitted to 
Professor Blum, of Heidelberg, who was the first to examine its 
composition*. This distinguished petrologist at once recognized it 
as a true nepheline-bearing syenite ; and so struck was he with the 
close resemblance it bears, not only to certain Norwegian syenites 
(often erroneously termed zirconia-bearing syenites), but also to a 
syenite occurring in one of the Cape-Verd islands, that he united 
them all into a separate group called Foyaites. Of these, however, 
the Foyaite proper is so rarely met with that, to the large majority 
of petrologists, it 1s almost terra incognita ; nor is the locality itself 
easily accessible to geological research. J have been enabled to 
examine under the microscope more than 40 sections of this rock 
representing different parts of the locality; and so interesting and 
varied is its composition, that it may not be inopportune to give a 
description of it, which I shall preface by a rapid sketch of the 
leading features of the Foya district, as iulustrated by the annexed 
plan and section (Plate I.). 
The range of hills which forms the northern boundary of the 
ancient province of Algarve, in the south of Portugal, may be 
regarded as a continuation of the Sierra Morena in Spain, inter- 
cepted by the river Guadiana. It consists chiefly of Devonian grey- 
wacke, slates, and sandstones, and, striking west, reaches nearly to 
the Atlantic, whilst its numerous southerly spurs come in contact 
with Jurassic limestones and Cretaceous strata as they approach the 
coast-line. It is in the north-western part of Algarve, viz. in the 
Sierra de Monchique, that from the greatly disturbed and contorted 
greywacke strata there rises an extensive outcrop of crystalline 
rock in the shape of two flat, elongated domes—the Foya and the 
Picota. They are the highest points south of Lisbon, their altitudes 
being 905 and 735 metres, or 2968 and 2410 feet respectively t. 
The point of contact with the greywacke strata lies at contour 182: 
hence the vertical outcrop of the crystalline mass is equal to 726 
metres, or about 2400 feet, whilst the superficial area it covers 
amounts to about 84 square miles. It is a noticeable feature that 
the Picota does not follow the general direction of strike, and that 
its longitudinal axis deviates from that of the Foya by an acute 
* Jahrbuch fiir Mineralogie, 1861, p. 426. ‘The specimens were collected by 
Dr. Reiss. 
+ The gerade are taken from General Folque’s Ordnance Map of eee 
scale O10 000° 
