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We present observations of velocity-resolved [CII] 158 $\mu$m emission fromboth a dense and a more diffuse photodissociation region (PDR) in the Perseusgiant molecular cloud (GMC) using the Heterodyne Instrument for theFar-Infrared onboard the Herschel Space Telescope. We detect [CII] emissionfrom 80\% of the total positions, with a 95\% detection rate from the denseboundary region. The integrated intensity of the [CII] emission remainsrelatively constant across each boundary, despite the observed range in opticalextinction between 1 mag and 10 mag. This flat profile indicates a constantheating and cooling rate within both regions observed. The integrated intensityof [CII] emission is reasonably well correlated with the neutral hydrogen (HI)column density, as well as total gas column density. This, in addition to the80$'$ (7 pc) extent of the [CII] emission from cloud center, suggests that theHI envelope plays a dominant role in explaining the [CII] emission emanatingfrom Perseus. We compare the [CII] and $^{12}$CO integrated intensities withpredictions from a 1-D, two-sided slab PDR model and show that a simple core$+$ envelope, equilibrium model without an additional "CO-dark" H$_2$ componentcan reproduce observations well. Additional observations are needed todisentangle how much of the [CII] emission is associated with the "CO-dark"H$_2$ gas, as well as constrain spatial variations of the dust-to-gas ratioacross Perseus.

Herschel 158$ \mu$m [CII] Observations of "CO-dark" Gas in the Perseus Giant Molecular Cloud